This invention relates to a system for the separation of C.sub.2+ or C.sub.3+ hydrocarbons from a hydrocarbon-containing raw gas under pressure, and in particular, wherein the raw gas is cooled and expanded and the resultant condensates are introduced into a rectification column wherein the C.sub.2+ and C.sub.3+ hydrocarbons are separated from the lower-boiling components.
Such a process, with the particular objective of obtaining C.sub.2+ hydrocarbons, has been proposed in assignee's German Patent Application No. P 33 32 943.5 and corresponding U.S. application Ser. No. 650,016, filed Sept. 13, 1984, by Kummann et al, incorporated herein by reference. In this earlier patent application, the gas fed to the column and in some cases the gas withdrawn from the column as well, are subjected to different pressure stages which in turn facilitates heat transfer between separate streams. In a specific embodiment, a methane fraction is produced during the cooling of the crude gas by, in essence, cascade fractional condensation steps. This methane fraction is so depleted in C.sub.2+ hydrocarbons that there is no need to introduce it into the rectifying stage. Accordingly, the fraction is not expanded to the rectification pressure and consequently, requires little or no compression before being discharged into a high-pressure methane pipeline.